Note the "Number of accounts: 1" instead of the expected 52. The size of the backup as reported by a "du" command is also around 500KB for the broken ones instead of the usual ~30GB of the other full backups. The backup process also takes around 15 seconds instead of an expected 30 minutes.

Incremental backups on the other hand always seem to work fine.

The disk is nowhere near full so that can't be an issue.

The backup system is not something you want to be flakey! Are error conditions reported correctly by the backup system?